As a type of high-energy density secondary battery, there has been proposed a non-aqueous electrolyte-type lithium secondary battery using a carbon material as a negative electrode (e.g., Patent documents 1-4 listed below). The battery utilizes a phenomenon that a carbon intercalation compound of lithium can be easily formed electrochemically, and when the battery is charged, lithium in the positive electrode comprising, e.g., a chalcogenide compound such as LiCoO2, is electrochemically inserted between carbon layers in the negative electrode (doping). The carbon thus doped with lithium functions as a lithium electrode to cause a discharge, whereby the lithium is liberated (dedoped) from the carbon layers to return to the positive electrode. Such a non-aqueous electrolyte secondary battery is small-sized and light in weight to have a high energy density, and accordingly demands therefor have increased as power supplies for portable appliances.
As negative electrode materials for such non-aqueous electrolyte secondary batteries for portable appliances, there has been also widely used non-graphitizable carbon having a potential capacity well exceeding a theoretical value of 372 mAh/g of graphite in terms of discharge capacity per 1 g of carbon, and in order to increase the discharge capacity thereof, several measures have been taken such as to effect a calcination while positively flowing an inert gas (Patent document 5 below) or under a reduced pressure (Patent document 6) so as to positively remove gas generated during carbonization reaction, thereby accelerating the growth of pores. However, the thus-formed non-graphitizable carbon is accompanied with a problem that it is oxidized when left in the atmosphere to result in an increase of irreversible capacity and a lowering in cycle characteristic. For solving the problem, there has been proposed a method of storing non-graphitizable carbon in an inert gas atmosphere (Patent document 7 below). Further, as another means for suppressing the degradation, there has been also proposed a method of depositing pyrolyzed carbon in the carbon surface to adjust the pore diameter (Patent document 8 below).
As a new use taking advantage of the characteristic of small-size and lightness of the non-aqueous electrolyte secondary battery, the development of batteries for electric cars or vehicles, such as electric cars (EV) driven by only motors and hybrid-type electric cars (HEV) combining an internal combustion engine and batteries, has been extensively made, and particularly HEV combining an engine and a battery-driven motor has been put before footlights as a motor car satisfying economy and low fuel consumption. Particularly, since the appearance as a commercially available car, an HEV has been increasing its markets because its concept of amenity to environments. For this type of cars, there has been demanded a weight reduction of batteries and an improvement in battery performances represented by input and output densities that directly lead to an improvement in fuel consumption, and a part of non-aqueous electrolyte secondary batteries have been commercialized for car use.
An HEV car is driven by increasing the load of a motor having a better drive efficiency than an internal combustion engine in a drive or running mode of a low velocity or a low load and driven by principally using the engine having a better drive efficacy than the motor in a drive mode of a high speed or a high load, such as quick start or passing at a high speed. Further, at the time of braking, the motor is used as a dynamo to recover a regenerated current and store it in the battery. Accordingly, the motor is principally used at the time of start, acceleration and speed reduction, and the battery for supplying an electric power to the motor is required to exhibit not a high energy density for supplying a small power for long hours but high input and output performances required for repetition of supply and receipt of large powers in a short time.
Thus, a secondary battery for HEV is required to show performances which are different from those of conventional secondary batteries for small-size portable appliances. However, the studies hitherto made for negative electrode materials for non-aqueous electrolyte secondary batteries have been almost exclusively directed to improved performances for negative electrode materials for secondary batteries as power supplies for small-size portable appliances. As a result, it is a present state that a negative electrode material having sufficient properties suitable for non-aqueous electrolyte secondary batteries of high input and output currents as represented by secondary batteries for HEV has not been developed.
Patent document 1: JP-A 57-208079
Patent document 2: JP-A 62-90863
Patent document 3: JP-A 62-122066
Patent document 4: JP-A 2-66856
Patent document 5: JP-B 3399015
Patent document 6: JP-B 3427577
Patent document 7: JP-A 8-298111
Patent document 8: JP-A 2003-323891